A definite noun in English takes the definite article the, and an indefinite noun takes the indefinite articles a or an. As far as indefinite nouns are concerned, things are very similar in Swedish. The indefinite articles are en and ett. A neuter noun takes ett and a common noun takes en.
I'm fairly sure that "ett" and "en" both mean "a/an" and the gender of the word decides which you use (e.g "en hund" is "a dog" and "ett bord" is "a table"). When you want to say "the", it goes onto the end of the word ("hunden" would be "the dog"). Sorry if that's a bit confusing - I'm new to this myself!